Method of coating metal



Patented Jan. 5, 1937 PATENT OFFICE 2,067,007 Mn'mon or ooarmc METAL Van M. Darsey, Detroit, Mich}, assignor, by mesne assignments, to The Patents Corporation, De-

troit, Mich, a corporation of Michigan,

,No Drawing. Application March 5, 1934,

Serial No. 714,098

- 4 Claims. (01. 148-6) ilThis invention relates to a method of coating *1 metal and more particularly to producing on metal a coating consisting largely of pyrophosphates by means of a solution of pyrophosphates. 5 An object of the invention is to obtain a pyrophosphate solution capable of. forming a pyrophosphate coating on metal. Another object of the invention is to produce a pyrophosphate aqueone solution which remains indefinitely in the pyro form. -More specifically one object of the invention is to maintain a pyrophosphate solutlon in the pyro form by the use in the solution of apyrophosphate salt of one or more of certain metals. "Further objects anddetails will appear asthe description proceeds.

I It has. been known that dilute ortho-phosphate solutions may be employed to produce phosphate coatings on iron, steel, zinc and magnesium. It

has been found that pyrophosphate solutions may be ,Slmilarly employed with proper modifications.

One difliculty encountered is that an acid pyrophosphate produces more free acid by hydrolysis in a dilute aqueous solution thandoes a similar amount of ortho-phosphate, and this involves some Hdifliculty in balancing the bath. Another difilculty is that many pyrophosphates quickly revert to the ortho form when dissolved in water, even at ordinary room temperatures, and pyrophosphates which do not revert at room temperature are quickly changed to the ortho form when boiled.,

It has been discovered that acid pyrophosphate of zincand of the alkali metals and oi the alkaline earth metals, when dissolved in water, hydrolyz to produce varying amounts of pyrophosphoric acid in the solution and that, with the presencein the solution of the phosphates mentioned, the free pyrophosphoric acid remains indefinitely in the pyro form at ordinary tempera- 40 tures. The amount of ffree acid liberated is in general greater with the pyrophosphates than with the similar ortho salts.

The principal use of the phosphate coating method has been to coat articles of iron or steel in phosphate-baths containing acid phosphates of iron, zinc and manganese. 'It has been pro--' posed to employ also calcium phosphate, but t has not been commercially satisfactory.

In using acid phosphate solutions, it is found that pyrophosphates of iron or manganese quick ly revert to the ortho form, so thatthe results,

are essentially the same whether the bath is formed initially from pyrophosphates of mangaynese or iron or from ortho phosphates of these metals, but with a solution formed from zinc pyrophosphate, the salt remains in the pyro form at ordinary temperatures and even at somewhat elevated temperatures, although it is transformed to the orthoform by boiling. Taking advantage of this fact, it is found that, by accelerating the 5 action of the solution, so that it coats at a temperature-below that at which it changes to the ortho form, a pyrophosphate coating may be produced upon iron, steel, zinc or magnesium.

The action of the solution may be accelerated by the use of an oxidizing agent or by the use of a salt of a metal below the metal to be coated in the electromotive series, or by both. Examples of solutions which have been found operative are given below. i

A solution having approximately the proportions of 15 grams zinc acid pyrophosphate. 2 grams ferrous sulphate and 2 grams tartar emetic in 500 cc. of water produces a good paint-holding coat on zinc or galvanized iron in from 2 to 5 minutes at ordinary room temperatures.

The acid pyrophosphate given in the above formula is understood to be the dihydrogen pyrophosphate, which may be formed by heating the dihydrogen ortho-phosphate.

Two grams of cadmium sulphatelmay be used in place of the it grams of; tartar emetic, or one gram or copper sulphate may beemployed. It will be understood that other salts of antimony, cadmium or copper may be used, and that also salts of other metals below zinc in the electromotive series may be employed, although the metals mentioned are excellent for the purpose.

Such oxidizing agents as nitrates are beneficial when the accelerating metals are iron and copper, but tend to retard the action when antimony o'r cadmium are employed as accelerating agents.

For coating iron, a suitable solution may be formed by employing 15 grams zinc acid pyrophosphate, 7 grams sodium nitrate and a trace of copper in 500 cc. of water. The free acid resulting from the use of zinc pyrophosphate is so high that copper is plated out very quickly and, in order to avoid a smudgy coating resulting from an excess ofcopper, it is necessary to keep the copper content lower than would be required in an otherwise similar ortho-phosphate solution, or else diminishing the proportion of free acid in the solution by other means.

As suggested above, thepyrcphosphates of the alkali metals and of the alkaline earth metals likewise result in solutions wherein the pyrophosphoric acid remains in the pyro form at ordinary temperatures, but none 01' these hydrolyze to so great an extent as does zinc pyrophos- 55 phate. Magnesium pyrophosphate, for example, hydrolyzes to produce about the same amount of free acid in the solution as is produced by dissolving therein an equal amount of zinc orthophosphate, which is about one-half as much free acid as is produced by the zinc pyrophosphate. Sodium pyrophosphate, for example, produces very little acid. If desired, for purposes of allowing greater range in the copper content or for other balancing purposes, some or all of the pyrophosphate may be introduced as salts of the alkali metals or alkaline earth metals instead of the zinc phosphate, but whatever salts are used, there must be sufiicient hydrolyzation obtained to result in the coating operation.

In considering accelerating metals suitable for use with iron, it will be understood that cadmium, which is very helpful with zinc, is of comparatively little importance with iron. Furthermore, compounds of antimony are not helpful in coating iron and appear to act somewhat in opposition to the accelerating action of the oxidizing agent. Decided acceleration of the action must be obtained with iron in order to secure proper coating action at such a reduced temperature that the pyrophosphate is not transformed to ortho-phosphate. This can be best secured by use of both an oxidizing agent and a permissible amount of copper.

by the use of phosphates of zinc, the alkali metals or the alkaline earth metals. Magnesium is much more easily coated in the same manner and, therefore, no detailed description of methods of coating magnesium is required. It will also be understood that the formulas given above for coating zinc and iron are given by way of example only, and that other formulas may be prepared by using equivalents of the chemicals mentioned.

While the excess acidity of the pyrophosphate solution over the corresponding ortho-phosphate solution introduces some difficulties in properly balancing the solution for satisfactory coating operation, it likewise has the advantage of attacking the metal very promptly and it will act upon slightly rusted iron or steel or upon weathered zinc somewhat more satisfactorily than corresponding ortho-phosphate solutions.

It is sometimes diflicult to avoid forming objectionably large crystals in a coating produced by an orthophosphate solution. A coating with such large crystals, customarily called sparkle, is notas rust-proof as a fine grained coat, or as smooth under paint. The use of a pyrophosphate solution substantially eliminates trouble with sparkle.

Other advantages of producing the pyrophosphate coating instead of the ortho-phosphate coating will be readily apparent.

'Having indicated some specific examples of the invention and having also indicated its general scope so that others skilled in the art may employ the invention successfully, what is claimed is:

1. The method of forming a phosphate coating comprising a material amount of pyrophosphate on a metal of the group consisting of iron, steel, zinc, magnesium and their alloys, which comprises subjecting the surface of said metal to a dilute acid pyrophosphate aqueous solution, at a temperature below that at which the pyrophosphate changes to orthophosphate in the solution employed, and including in the solution an acid phosphate of a metal of the group consisting of zinc, the alkali metals and the alkaline earthmetals.

2. A method in accordance with claim 1, and comprising accelerating the action of the solution by an oxidizing agent.

3. A method in accordance with claim 1, and comprising the acceleration of the'action of the solution by the use in the solution of a soluble compound of a metal below the metal to be coated in the electromotive series.

4. A method in accordance with claim 1, and in which the metal coated is iron or steel and the solution is of zinc acid pyrophosphate and approximately one-half as much sodium nitrate as zinc acid pyrophosphate, and the solution is maintained at a temperature below boiling.

VAN M. DARSEY. 

